The present invention relates to a diaphragm used with a master cylinder, and more particularly to a diaphragm assembly adapted to be fitted to a reservoir so as to cut off the communication between a working fluid filled therein and the outside air.
As well-known in the art, the diaphragm of this type used with a master cylinder is fitted to a reservoir with its outer periphery being engaged with a reservoir cap and fixed to the reservoir so as to, on the one hand, prevent the working fluid from being contaminated with the atmosphere and, on the other hand, prevent any negative pressure from being generated following a drop in the level of the working fluid. To ensure that the diaphragm follows a drop in liquid level, the diaphragm is provided with a bellows portion. For instance, it has been proposed to form the diaphragm with a bellows portion in the radial direction of an associated reservoir. However, it is likely that an amount of air is confined within the spaces between the lower corrugations of the bellows portion, mixed with a working fluid due to the vibration of a vehicle body, and guided into a pressure system during braking, thereby causing a so-called vapor lock phenomeneon.
To this end, a radially opening air passage is provided through the spaces of the lower corrugations of the bellows to drive air out of the reservior. With that air passage being flat and only passing through part of the spaces defined by the corrugated portion, however, it is impossible to remove the air confined within the said spaces in a reliable and smooth manner. Provison of the air passage to part of the bellows portion also leads possibly to the generation of a negative pressure, since the diaphragm does not satisfactorily drop to the liquid level.
To make it more reliable for the diaphragm to follow a drop in liquid level, it has also been proposed to provide the reservoir with a bellows portion in its axial direction. Owing to the absence of any air-confining space, there is no fear that air may be carried into a pressure system. However, mold making is complicated with rises in the production cost as a consequence.
With the master cylinder, difficulties would be encountered in increasing the pressure in the pressure system, if the amount of liquid in the reservoir is decreased.
To detect the amount of liquid, therefore, a viewing window(s) is provided in the reservoir in the case that it is formed of an opaque material, and united with the body of the master cylinder, or alternatively the reservoir per se is made of a transparent material.
When the diaphragm is formed with a bellows portion, however, it begins to hang down at its central tip with a decrease of the amount of a working liquid, thereby forcing the working liquid into the space between it and the inner wall of the reservoir, or closing up the window. This poses a problem in connection with the accurate dectection of a liquid level.